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28t or 30t Chainring

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28t or 30t Chainring

Old 06-10-18, 08:49 PM
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bajajoaquin
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28t or 30t Chainring

I just bought a bakfiets with a Sturmey Archer XFRD-8 hub. Thatís the one with first being direct and 8th being a 300% overdrive. It has the 23t rear sprocket and a 36t chainring. I would like to run a 25t rear and 28t front. I donít know the bolt circle diameter, but it doesnít look like the one from the inside ring of a triple. It looks like the outer diameter.

How do do I measure the bolt spacing?

is a 28t or 30t available on such a bolt circle?
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Old 06-10-18, 08:58 PM
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The way to measure your BCD or bolt circle diameter is to measure from the center of one bolt hole to the center of an adjacent bolt hole.

Here is an online calculator to tell you what your BCD is, just plug in the measurement in millimeters and the bolt pattern (4 or 5 bolts). Until we know what your BCD is there is no way to know if a 28T or 30T chainring is available. Just keep in mind that the larger the BCD number, the bigger the smallest chainring size it can be.
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Old 06-10-18, 09:07 PM
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Originally Posted by bajajoaquin View Post
. . . It has the 23t rear sprocket and a 36t chainring. I would like to run a 25t rear and 28t front. . .
That will increase input torque by 40%. You should probably observe manufacturer's limits.
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Old 06-10-18, 11:48 PM
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Originally Posted by bajajoaquin View Post
I just bought a bakfiets with a Sturmey Archer XFRD-8 hub. Thatís the one with first being direct and 8th being a 300% overdrive. It has the 23t rear sprocket and a 36t chainring. I would like to run a 25t rear and 28t front. I donít know the bolt circle diameter, but it doesnít look like the one from the inside ring of a triple. It looks like the outer diameter.

How do do I measure the bolt spacing?

is a 28t or 30t available on such a bolt circle?
So your stock gearing is 36T front x 23T rear. You would like to run 28T front x 25T rear. Why? Not that it matters, it probably can't be done. Not with that crank anyway. If it doesn't look to you like a 64mm or 74mm bolt circle typical for granny ring tooth counts in the low 20's then you have something larger and you will be hard pressed to get much below the 36T you have now.
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Old 06-11-18, 10:10 AM
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Originally Posted by *Scuba View Post
The way to measure your BCD or bolt circle diameter is to measure from the center of one bolt hole to the center of an adjacent bolt hole.

Here is an online calculator to tell you what your BCD is, just plug in the measurement in millimeters and the bolt pattern (4 or 5 bolts). Until we know what your BCD is there is no way to know if a 28T or 30T chainring is available. Just keep in mind that the larger the BCD number, the bigger the smallest chainring size it can be.
Thank you. That calculator was exactly what I was looking for. Of course, once I had calipers out to measure that, I just measured from the center of the cranks to one bolt and doubled it. Doh.

It's 110m

I'm seeing 34t readily available. Still curious how much smaller is out there. A 34-25 would have the advantage of keeping the same chain, but would only be dropping down 14%, which is one gear on this hub.

As for the why: I said it in my original post. It's a bakfiets and the hub has 1:1 low. It's a heavy bike, and you can't really mash up hills, because of the way it is set up.

Sheldon Brown's site recommends that you have your regular riding gear be in the middle or top end of the available range. Right now, second and third gears are that gear. Turning onto my street, I'm in second gear, and only have one gear to drop down climbing the hill to my house, and no more gears down to climb the grade into my driveway. I'm trying to get to a 28% drop in ratio so I can drop two gears.

Last edited by bajajoaquin; 06-11-18 at 10:14 AM. Reason: added bcd
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Old 06-11-18, 11:30 AM
  #6  
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one thing about the S-A 8 speed , its lowest gear is the 1:1, cog driving the hubshell directly, no reduction gears .

the Other 7 are all overdrive gears IDK its Weight handling capacity, but is I see it ,

the drive ratio minimums are usually associated with internal reduction gears ..

Many hubs are symmetrical the 1 is the middle gear ,
and the reduction gears an inversion of the overdrive gears

like the venerable 3 speed, low is 3/4, 0.75, high is 4/3, 1.33..

Sachs used to make a Utility 5 speed, before Sram Bought them, and closed all IGH making
for the trend to high profile 1 by and race bike stuff.

reading NuVinci has a heavy duty version of their continuously variable ratio hubs , for electric mid drive bikes

If that hub won't hold up, to your application, it may be a consideration.



...
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Old 06-11-18, 12:32 PM
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You're exactly right. I was both happy and disappointed when I checked out the bike and it had the 8-speed. Happy that it had an 8-speed hub with 300% gear range and a drum brake (X-RD8), but disappointed in that it was this one with the odd gearing.

Had I been upgrading to a multi-gear hub, I was going to consider the RX-RD5 which is symmetrical as you said. I also like the NuVinci, but I don't think they offer a drum brake (which I admit I'm just kinda stuck on). I'm not a fan of the Shimano roller brakes.

However, since it's several hundred dollars to upgrade the hub, but only $30 or $50 to upgrade chain and sprockets, I'm going to try this route first.

Incidentally, my other happy/sad moment was when I saw it had a Nexus generator hub and lights. There's more reason for a front brake upgrade because it has really bad caliper brakes now, and the lights don't currently work. If I can get them working by fiddling with connections, then I will see what I can do with the brakes. Otherwise I had considered the SA XL-FDD 90mm brake/dynohub. But again, if this works after some fiddling, I really can't justify a couple hundred bucks because I have a soft spot for the Sturmey Archer brand.
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Old 06-11-18, 01:19 PM
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the other drum brakes are all S-A maybe 3,4*, or 5,
or drop the IGH they offer a Cassette Drum and a screw on freewheel drum.
(4 speed is 3 of 4 overdrive gears, vs your 7 of 8.)

FWIW, I have been using a 6 speed freewheel drum brake hub since the 80's
but on a single bike ..

They used a 9mm axle, front and rear ..


Sachs , while still in business in Germany did offer a drum brake..





...

Last edited by fietsbob; 06-11-18 at 01:32 PM.
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Old 06-11-18, 01:40 PM
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110 bcd limits you to 34 t smaller you need another crankset
like just using the granny gear of a triple..

there are a few internally geared cranks offering a 27/28t, &
​​​​​​​over driven 1.6x, German-Swiss one has Cargo strong options..
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Old 06-11-18, 02:33 PM
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A local bike shop (who can't compete with Amazon, so he just performs service, incidentally) Said he's got an old mountain bike crankset that will fit. If 25/34 doesn't get me there, I'll think about going with the inner ring on a triple.

Thanks for the guidance.
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Old 06-11-18, 05:04 PM
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Originally Posted by bajajoaquin View Post
Thank you. That calculator was exactly what I was looking for. Of course, once I had calipers out to measure that, I just measured from the center of the cranks to one bolt and doubled it. Doh.

It's 110m I'm seeing 34t readily available. Still curious how much smaller is out there.
That's it. 34T is your low limit with that crankset which is what I suspected. Thing is, the people that make your bike know that it is a precious (and heavy) cargo carrier and they spec'd that hub (and crank for it). I absolutely know what that's worth, but still I have to wonder. I mean... on a 'normal' bike you would want 1:1 to be equal to the wheel size, if not a little smaller. On your bike you would want 1:1 to be a serious underdrive. That you can pedal it at all in the 1:1 makes me wonder if you are really correct about that hub and its setup. In any case, are there not bigger rear cogs for the 8-sp IGH? Yes, you will need a new chain, but that is a trivial cost compared to getting an electric assist which is how I see a lot of the bakfiets in my neighborhood set up. The usual rules for how to set up gearing kind of go out the window with your particular platform. You are likely to spend a lot of time at the very lowest end of the speeds range, much more than you might think is normal.
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Old 06-11-18, 05:39 PM
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A 110 mm BCD can accept a 33T chainring at the smallest but 34T chainrings are far more common.
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Old 06-12-18, 08:58 AM
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But .. Only TA of France, makes the 33t,

And it requires removing metal from the ends of the crank arm spider,
for clearance.. or the chain wont settle fully on the chainring tooth..


1/8" chain? there are some small chainring capable 3 piece BMX cranks to check out. https://www.danscomp.com/bmx/Parts/Cranks/3-Piece/
https://www.danscomp.com/bmx/Parts/Sprockets/
no input on the chainline .. FWIW





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Old 06-12-18, 01:27 PM
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I went with a 3/32" chainring, figuring the gap of 0.03125" was small enough to not worry about. I'll stick with the 34 for now, as I don't really want to file the ends of the spider. If I need to go lower, I'll use the inside of a triple. Or buy a different hub.

On to the Nexus hub generator!

Thanks for the help.
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